What may be causing some unpleasant surprises with UC applications is overconfidence and overreaching, driven by the following:
A. Comparing exaggerated weighted GPAs to UC-recalculated weighted-capped GPAs reported in UC admission stats. Many prospective UC applicants on these forums seem to be unaware of the UC weighted-capped GPA calculation that UC web sites generally use unless otherwise stated. So a student whose high school weighted GPA is 4.4 off of an unweighted 3.6 may feel that s/he has a good chance at UCB based on UCB’s 25th-75th GPA range of 4.15-4.30. But his/her true chances may be much worse if his/her UC weighted-capped GPA is more like 3.9.
B. Overreliance on test scores to assess chances, when UCs tend to consider GPA more heavily than test scores. Applicants who are “test score heavy” are especially likely to have disappointing results if they base their reach/match/safety assessments of UCs on their test scores.
C. Competitive majors (commonly CS and engineering majors, but can also be other majors, depending on the campus) can have significantly higher admission selectivity than the campus overall. Applicants aiming for competitive majors may need to aim lower when making their application lists (this is not specific to UCs).